Hello and welcome to the forum. Well done for giving a Bramble a new home. It sounds as if you have made a good start.
There are many health problems that can arise due to obesity so it is good that you are trying to address this. We have quite a few threads on the forum about this and having just searched using the term 'weight loss' I have found the following but there are more:Obesity and weight loss threadWeight lossNeed a weight loss food and fast
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Essential Contour is a good quality food and has a nutritional rating of 91% - link.
Protein is above average, fat is average and carbohydrate is above average. Butternut Box is also a good quality fresh food and can be fed as the whole diet or part in conjunction with other products. Both are complete foods.
My personal feeling is that with a dog who needs to lose weight it is best to keep it simple and just feed one type of complete food. If it is weighed properly and the dog is not given anything else it will quickly become apparent if the quantity is correct and you can adjust by 10% as required. Dry food can be enhanced by using suitable (non starchy) cooked, mashed vegetables and it will help to sate the dog's appetite. More information here.
3kg is a lot to lose so take your time and aim for a steady weight loss. It is absolutely essential to weigh the food accurately so that you can reduce it gradually (by 10%) if weight loss does not occur. Also, only give the dog it's rations and no extras. If you wish to give treats, reduce the food a little. Don't over exercise your dog in an attempt to encourage weight loss. It can be too much of a strain on the dog. Just aim for gentle, frequent walks at the moment. Diet will reduce weight better than trying to get it off by over exercising them, particularly strenuous exercise.